Infection by the HIV virus causes a change in a number of physiological processes, the etiologies of which are poorly understood. Included in this are the depletion of T cells expressing CD4 and B cells expressing immunoglobulins, and changes in the regulation of cytokines. Crucial to an understanding of these processes and to developing therapeutic strategies related to these changes is the determination of the structural motifs critical to the physiological processes involved in the changes. We are employing the methodologies we have been using for epitope determination (protection assays combined with mass spectrometry) (1-4) to probe receptor-ligand pairs relevant to HIV infection including: a) HIV gp120 and immunoglobulin; b) CD4 and gp120 vs. CD4 and Major Histocompatibility Complex class II (MHC-II) molecules, and; c) tumor necrosis factor alpha (TNF') and monoclonal antibodies. Our initial experiments have been to probe uncomplexed gp120 in solution using a variety of enzymes. We found that the recombinant gp120 from CHO cells we had intended to use is extremely heterogenous. Recombinant gp120 was obtained from the AIDS Research and Reference Reagent Program and was found to be significantly purer. Experiments are underway with this material to characterize the products of its proteolysis.